You can also fix this by adding a bias resistor from the base of the
PNP transistor to ground and feeding the audio into the midpoint of the
diodes with an isolating capacitor.
Using Q71 as a current source for Q72 and Q73 will require either
negative feedback or an adjustable resistor in the Q71 base bias or
even both, otherwise differences in the transistors used can cause
problems. You may even need to add a balance resistor between the bases
of Q72 and Q73 to eliminate cross-over distortion.
With my solution these adjustments aren't needed as much.
On Fri, 29 Jun 2018 19:02:34 -0700
"ajparent1/KB1GMX" <kb1gmx@...> wrote:
The problem is if the first transistor Q71 is not biased to the half
power supply point the output are both unbalanced and the bias is
wrong. IF Q71 collector is not at about 6V (for 12V supply) full
stop. Adjust R72 until it is. Then adjust the bias if needed to the
The most common variation is to derive the bias from the common point
of both emitters as then it tries to self center at 6V or half the
supply. The cost of of doing that is lower gain but higher fidelity.
See the design from this site (attached) and explanation.
Do not let their published 3V power scare you it works well at 12V
too. The key is how Q1 is biased and the resulting bias to the output
string (3904 and 3906).
It should be possible to change the V4 circuit to that.